A technique is known, in which a mat composed of an insulator is provided between a catalyst carrier which generates the heat by applying the electricity and a case which accommodates the catalyst carrier (see, for example, Patent Document 1). According to this mat, it is possible to suppress the electricity from flowing through the case when the electricity is applied to the catalyst carrier. However, when the particulate matter (PM) contained in an exhaust gas adheres to the mat, it is feared that the electricity may flow through the case by the aid of PM. On the other hand, water contained in an exhaust gas is sometimes condensed on a wall surface of an exhaust tube, for example, immediately after the startup of an internal combustion engine. Water, which is in a liquid form, is allowed to flow toward the downstream side by being pushed by the exhaust gas, and water arrives at the catalyst. If water in the liquid form invades the inside of the mat, and water arrives at an electrode, then the insulation resistance between the electrode and the case is lowered. Therefore, it is feared that the electricity may flow from the electrode to the case.
On the other hand, a technique is known, in which an electric heater is arranged along an outer wall of an exhaust tube of a vacuum apparatus, and a heat resistant vacuum heat insulating material is wound around the outer circumference of the electric heater (see, for example, Patent Document 2).
Further, a technique is known, in which the air is allowed to exist between an inner tube and an outer tube of a double tube so that the temperature of an exhaust gas introduced into a catalyst is thereby maintained at a high temperature to improve the exhaust gas purification performance, and the increase in the surface temperature of an exhaust tube is suppressed (see, for example, Patent Document 3).